Method for handling maleimides

ABSTRACT

A method for handling a maleimide, which method comprises handling said maleimide in the presence of a stabilizer at a temperature exceeding the melting point thereof in a state kept in contact with a gas phase having a molecular oxygen content of not more than 10% by volume.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for handling maleimides. Moreparticularly, it relates to a method for handling, as in the process oftransportation and storage, maleimides fast in a molten state proofedagainst coloration. Still more particularly, it relates to a method forsafely and easily handling maleimides in a state proofed againstcoloration and polymerization.

2. Description of the Prior Art

The maleimides are compounds which are useful as raw materials forresins, medicines, and agricultural pesticides.

Heretofore, the maleimides which are solid at normal room temperaturehave been generally handled in the form of powder, flakes, and tablets.The maleimides in such a form entrain minute particles of themselves.Particularly during the transportation of such solid maleimides, themaleimides undergo gradual comminution and give rise to minute maleimideparticles in a large amount.

The maleimides themselves possess a property of stimulating the humanbody. Particularly when a maleimide in the form of a fine powder isinhaled by a man, it stimulates the nasal cavity and the throat of theman and compels him to cough or sneeze. When this maleimide powder issuffered to adhere to the skin of a man and remain thereon, it causesinflammation of the affected area of his skin. The maleimides possessthis undesirable quality. A man engaging in the work of handling amaleimide containing minute particles of itself, therefore, must payrigid attention to keeping his skin from contact with the maleimide tothe fullest possible extent.

Enormous labors, therefore, are required for the purpose of precludingthe occurrence of minute particles of a maleimide to the fullestpossible extent during the transportation of the maleimide or for thepurpose of removing minute particles from the maleimide after thetransportation.

When a solid substance is transported, it is packed in paper bags,drums, or other similar containers prior to the transportation in mostcases. When the solid substance is a maleimide in this case, a manengaging in the transportation inevitably exposes his body to themaleimide and consequently suffers from adhesion of minute particles ofthe maleimide to his skin.

An attempt at conveying a solid substance by a pipeline for the purposeof preventing the solid substance from contacting the human body isfundamentally difficult to implement. Since the solid substance inconveyance has the possibility of clogging the pipeline, for example,the particles of the solid substance are subject to exactingrestrictions as to shape, size, and specific gravity in order for thesolid substance to be safely conveyed.

Thus, it is only logical to conclude that the transportation orconveyance of a maleimide which is solid at normal room temperatureentails numerous problems. The same thing holds good with respect to themethod to be employed for the storage of a solid maleimide.

As means for the solution of the problems mentioned above, methods whichreside in handling maleimides in process of transportation or storage inthe form of a solution in acrylonitrile, styrene, or a (meth)acrylicacid ester have been disclosed in JP-A-62-162,1167(1987),JP-A-62-143,911(1987), JP-A-62-145,062(1987), andJP-A-63-(1988)-316,767. These method are free from the occurrence ofminute particles of maleimides detrimental to the human body and,therefore, may well be called excellent means as compared with themethod which handle maleimides in a solid state. When the maleimides arehandled in the form of a highly concentrated solution, these methodsrequire a high temperature for the reduction of solid maleimides intothe solution. Since the solvent to be used for the solution is acompound which in itself possesses an ability to polymerize, theproduced maleimide solution cannot be long retained intact at theelevated temperature. To be specific, the maleimide solution suffers theoccurrence of impuries such as the polymer of the solvent itself and thecopolymer of the solvent with the maleimide used as a solute. Theseimpurities cause coloration of the solution. This solution in itsunmodified form is subjected to adjustment of concentration and then, asa heatresistance improving agent for ABS resin, AAS resin, AS resin, orACS resin, for example, used for copolymerization with a monomerdestined to form such a resin. When the maleimide solution to be used inthis case happens to be colored as described above, the finished productis inevitably colored and consequently degraded heavily in commodityvalue.

The conventional methods which handle maleimides in the form of a solidor a solution, therefore, hardly deserve to be called perfect means.

An object of this invention, therefore, is to provide a novel method forhandling a maleimide.

Another object of this invention is to provide a method for safe andsimple transportation or storage of a maleimide without any possibilityof the maleimide producing minute particles of itself during the courseof transportation or storage.

Yet another object of this invention is to provide a method for handlinga maleimide in such a manner as to be prevented from coloration andallowed to retain its quality intact for a long time.

SUMMARY OF THE INVENTION

These objects are accomplished by a method for handling a maleimide,which method comprises handling said maleimide in the presence of astabilizer at a temperature exceeding the melting point thereof in astate kept in contact with a gas phase having a molecular oxygen contentof not more than 10% by volume.

These objects are further accomplished by a method for handling amaleimide, which method comprises melting the maleimide in the presenceof a stabilizer at a temperature exceeding the melting point thereof ina state kept in contact with a gas phase having a molecular oxygencontent of not more than 10% by volume, solidifying the resultantsolution by cooling, and heating the resultant solid maleimide to atemperature exceeding the melting point thereof in the presence of thestabilizer in a state kept in contact with a gas phase having amolecular oxygen content of not less than 0.1% by volume therebyallowing the maleimide to be handled in a liquid state.

We have continued a diligent study for the purpose of developing amethod for vesting a maleimide with thermal stability to resistcoloration when the maleimide is dissolved not by the use of a solventbut by being heated at a temperature exceeding the melting point thereofand is consequently handled in a dissolved state or a liquid state, tofind that the prevention of the maleimide from coloration aimed at bythis invention is attained by repressing the molecular oxygen content ofa gas phase enveloping the maleimide below a specific level. Thisinvention has been perfected on the basis of this knowledge. When themaleimide is dissolved by being heated to a temperature exceeding themelting point thereof, packed in the molten state in a container,solidified by being cooled and then transported or stored in the solidstate, and then put to use, the prevention of the maleimide fromcoloration aimed at is accomplished by again dissolving the solidmaleimide under specific conditions. In this case, since the maleimidewhich is solid at normal room temperature is not required to bedissolved by the use of a solvent or by being heated to an elevatedtemperature in preparation for storage, it can be stored intact for along time without any possibility of undergoing an alteration such aspolymerization. Thus, this invention may well be regarded as a literallysurprising advance in the art of handling maleimides.

The method of this invention, therefore, enjoys the followingadvantages.

(1) This method enables a maleimide which causes a strong stimulation inthe human body to be handled not in a powdery state but in a liquidstate and, therefore, allows safe and easy handling of the maleimide.

(2) It enables the maleimide to be stored long in the liquid statewithout a sacrifice of its high quality.

(3) When the maleimide stored in the liquid state is used in theproduction of a polymer, the finished product shows virtually no sign ofcoloration and enjoys high quality.

(4) When the redissolved maleimide is used in the production of apolymer, the finished product shows virtually no sign of coloration andenjoys high quality.

EXPLANATION OF THE PREFERRED EMBODIMENT

Now, this invention will be described more specifically below.

The maleimides which are effectively handled as in transportation orstorage by the method of this invention include N-methyl maleimide,N-ethyl maleimide, N-hexyl maleimide, N-octyl maleimide, N-dodecylmaleimide, N-benzyl maleimide, N-cyclohexyl maleimide, N-phenylmaleimide, N-nitrophenyl maleimide, N-methoxyphenyl maleimide,N-methylphenyl maleimide, N-carboxyphenyl maleimide, N-hydroxyphenylmaleimide, N-chlorophenyl maleimide, N-dimethylphenyl maleimide,N-dichlorophenyl maleimide, N-bromophenyl maleimide, N-dibromophenylmaleimide, N-trichlorophenyl maleimide, and N-tribromophenyl maleimide,for example. These are not the only maleimides for which this inventioncan be utilized.

The stabilizers which are effectively usable in the handling of amaleimide at a temperature exceeding the melting point thereof includemethoxybenzoquinone; p-methoxyphenol; phenothiazine; hydroquinone;diphenyl amines; methylene blue; zinc dimethyldithiocarbamate;dialkyldithiocarbamates such as copper dimethyldithiocarbamate andcopper dibutyldithiocarbamate; copper salicylate; thiodipropionicesters; mercaptobenzimidazole; alkyl-substituted hydroxybenzenes andalkylbisphenols; hindered phenols; phosphoric esters; phosphorousesters; and phosphoric acid amide, for examples. These are not the onlystabilizers which are available herein. These stabilizers may be usedeither singly or as a mixture of two or more members.

Particularly, at least one stabilizer selected from the group consistingof alkyl-substituted hydroxybenzenes, thiodipropionic esters, hinderedphenols, phosphoric esters, phosphorous esters, and phosphoric acidamide is highly effective in preventing a maleimide from coloration at atemperature exceeding the melting point of the maleimide. Thus, thesestabilizers are advantageously used in this invention.

The use of at least one alkyl-substituted hydroxybenzene in combinationwith at least one phosphorus compound selected from the group consistingof phosphoric esters, phosphorous esters, and phosphoric acid amide as astabilizer brings about an outstanding effect in the prevention of amaleimide from coloration.

The alkyl-substituted hydroxybenzenes which are effectively usable as astabilizer in this invention include 2,4-dimethyl-6-tert-butyl phenol,4-tert-butyl catechol, 2,5-di-tert-butyl hydroquinone, 2-tert-dibutylhydroquinone, 4,4'-thio-bis(6-tert-butyl-m-cresol,2,4-bis(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine, 2,2'-thio-bis(4-methyl-6-t-butyl phenyl),triethylene glycol-bis(3-(3-t-butyl-5-methyl-4-hydroxyphenol)propionate),pentaerythrityl-tetra-kis(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate),octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate,2,2-thio-diethylene bis(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate),1,6-hexane diol-bis(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate),tris-(3,5-di-ti-butyl-4-hydroxybenzyl)-isocyanurate,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene,N,N'-hexamethylene-bis(3,5-di-t-butyl-4-hydroxy-hydrocinnamide), and3,5-di-t-butyl-4-hydroxy-benzyl phosphonate diethyl ester, for example.Among other stabilizers cited above, 2,4-dimethyl-6-tert-butyl phenol,4-tert-butyl catechol, 2,5-di-tert-butyl hydroquinone, 2-tert-dibutylhydroquinone, 4,4'-thio-bis(6-tert-butyl-m-cresol),2,4-bis(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine, 2,4'-thio-bis-(4-methyl-6-t-butyl phenol), andtriethyleneglycol-bis-(3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate) areadvantageously usable herein because they are excellent effective inpreventing a maleimide from coloration and from polymerization.

The thio-dipropionic esters which are effectively usable in thisinvention include ditridecyl-3,3'-thiodipropionate,dilauryl-3,3'-thiopropionate, ditetradecyl-3,3'-thiodipropionate,distearyl-3,3'-thiodipropionate, and dioctyl-3,3'-thiodipropionate, forexample. These are not the only thio-dipropionic esters which areavailable herein.

The hindered phenols which are effectively usable herein include2,4-bis(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine,2,2'-thiobis-(4-methyl-6-t-butyl phenol), triethylene glycolbis-(3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate),pentaerythrityl-tetrakis(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate),octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate,2,2-thio-diethylene-bis(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate),1,6-hexane diol-bis(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate),tris-(3,5-di-t-butyl-4-hydroxybenzyl)-isocyanurate,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene,N,N'-hexamethylene-bis( 3,5-di-t-butyl-4-hydroxy-hydrocinnamide), and3,5-di-t-butyl-4-hydroxy-benzyl phosphonate-diethyl ester, for example.Among other hindered phenols cited above,2,4-bis(n-aoctylthio)-6-(4-hydroxy-3,5-t-butyl anilino)-1,3,5-triazine,2,2'-thiobis-(4-methyl-6-t-butyl phenyl, and triethyleneglycol-bis(3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate) areadvantageously usable herein. These are not the only hindered phenolswhich are available herein.

The phosphorous esters which are effectively usable in the presentinvention include triphenyl phosphite, tris(nonylphenyl) phosphite,triethyl phosphite, tris(2-ethylhexyl) phosphite, tridecyl phosphite,tris(tridecyl) phosphite, tristearyl phosphite, diphenylmono(2-ethylhexyl) phoslphite, diphenyl monodecyl phosphite, diphenylmonodecyl phospite, dilauryl hydrogen phosphite, dilauryl hydrogenphosphite, diphenyl hydrogen phosphite, tetraphenyl dipropylene glycoldiphosphite, tetraphenyl tetra(tridecyl) pentaerythritol tetraphosphite,tetra(tridecyl)-4,4'-isopropylidene diphenyl phosphite, trilauryltrithiophosphite, bis(tridecyl)pentaerythritol phosphite,bis(nonylphenyl)pentaerythritol diphosphite, distearylpentaerythritoldiphosphite, tris(2,4-di-tertiary butylphenyl)phosphite, water-solublebis-phenol A, pentaerythritol phosphite polymer hydrated phosphenol A,and phosphite polymer, for example. These are not the only phosphorousesters which are available herein.

The phosphoric esters and other phosphorus compounds which areeffectively usable in this invention include hexamethyl phosphorictriamide, ethyldiethyl phosphonoacetate, ethyl acid phosphate,β-chloroethyl acid phosphate, butyl acid phosphate, butyl pyrophosphate,butoxyethyl acid phosphate, 2-ethylhexyl acid phosphate, di(2-ethylhexylphosphate, ethylene glycol acid phosphate, d(2-hydroxyethyl)methacrylate acid phosphate, tris(2-chloroethyl) phosphate,tris(dichloropropyl)phosphate, octyl dichloropropyl phosphate, phenyldichloropropyl phosphate, trimethyl phosphate, triethyl phosphate,tributyl phosphate, trioctyl phosphate, tricresyl phosphate, andtriphenyl phosphate, for example. These are not the only phosphoricesters and phosphorus compounds which are available herein.

The phosphoric acid amides which are effectively usable in thisinvention include hexamethyl phosphoric triamide, for example.

Among the phosphorus compounds cited above, at least one member selectedfrom the group consisting of tris(2-chloroethyl) phosphate,tris-nonylphenyl phosphite, tristearyl phosphite, distearylpentaerythritol diphosphite, and hexamethyl phosphoric triamide can beused advantageously.

The effect which the above mentioned stabilizer manifests in preventinga maleimide from polymerization is conspicuous as compared with thatwhich is manifested by p-methoxyphenol or hydroquinone conventionallyused as a polymerization inhibitor, as clearly demonstrated in theworking examples and controls to be cited herein below.

The amount of the stabilizer to be used in this invention is in therange of 0.0001 to 1.0% by weight, preferably 0.001 to 0.1% by weight.The kind of the stabilizer to be used herein is selected in dueconsideration of the kind of the polymer to be produced, the method ofpolymerization, the initiator to be used, for example. When analkyl-substituted hydroxybenzene and a phosphorus compound are jointlyused, though the ratio of these two compounds is not specificallydefined, the gravimetric ratio of the alkyl-substituted hydroxybenzeneto the phosphorus compound is desired to be in the range of 1:0.1 to1:1000, preferably 1:1 to 1:100.

When a maleimide particularly in a granular or powdery form is to behandled as heated to a temperature exceeding the melting point thereof,the method of this invention requires this maleimide to be handled asheld in contact with a gas phase having a molecular oxygen content ofnot more than 10% by volume. In accordance with our knowledge, adefinite relation exists between the coloration of a maleimide and themolecular oxygen content in the gas phase enveloping the maleimide beinghandled. In the normal atmosphere of air, the maleimide is colored at anotably high speed. Particularly in the absence of a stabilizer and inan atmosphere having a high molecular oxygen content, this coloration isaccelerated. When the maleimide is handled at a temperature exceedingthe melting point thereof, the repression of the molecular oxygencontent in the gas phase below the level of 10% by volume is effectivein preventing the maleimide from coloration. The desirability of theresult of handling increases in proportion as this concentrationdecreases. Particularly when the gas phase containing such an inert gasas nitrogen, carbon dioxide, or argon, the handling can be convenientlyeffected. In the gas phase, however, the molecular oxygen is preferableto be present in a concentration of at least 0.1% by volume. Theconcentration of the molecular oxygen present in the gas phase,therefore, is desired to be in the range of 0.1 to 10% by volume.

When the maleimide which has been dissolved as described above, thencooled until solidification, and stored or transported is put to use,the solid maleimide is handled still more effectively by being heateduntil liquefaction in the presence of the aforementioned stabilizer at atemperature exceeding the melting point thereof in a state kept incontact with a gas phase having a molecular oxygen content of not lessthan 0.1% by volume.

The method just described is carried out generally in a hermeticallysealed vessel. This hermetically sealed vessel is only required to beprovided with a heating part and may be in any desired shape. Thehermetically sealed vessels which are effectively usable herein includestationary tanks such as tanks furnished with an inner heating coil andtanks furnished with a heating jacket and mobile tanks such as tankcars, tank lorries, and bulk containers, for example.

A given maleimide is placed in a molten state in the hermetically sealedvessel. Though no specific condition is imposed on the work of placingthe maleimide in the vessel, the liquid maleimide is preferable to havethe aforementioned stabilizer incorporated therein in advance of theentry in the vessel. Inside the hermetically sealed vessel, the liquidmaleimide is solidified by either spontaneous cooling or forced externalcooling. Prior to use, the maleimide which has been solidified asdescribed above and then transported or stored is heated untilliquefaction. Then, the maleimide is handled in the resultant liquidstate.

In a preferred embodiment of this invention, a maleimide in a moltenstate is placed in a mobile tank such as a tank lorry or a bulkcontainer, solidified therein, transported in the resultant solid stateto a destination, heated there until liquefaction with a heating deviceannexed to the tank, and handled in the resultant liquid state.

A definite relation exists between the coloration and polymerizationwhich occur in a maleimide during the application of heat forliquefaction and the molecular oxygen content in the gas phaseenveloping the maleimide. The maleimide quickly polymerizes if themolecular oxygen content of the gas phase is less than 0.1% by volume.This polymerization is accelerated when the heating is carried out inthe absence of the stabilizer and the molecular oxygen content is lessthan 0.1% by volume. It has been found that the polymerization tends tobe repressed but the coloration tends to be encouraged when themolecular oxygen content is increased. For the method of this inventionto yield good results, the molecular oxygen content in the gas phase isrequired to exceed 0.1% by volume and desired to be not less than 0.1%by volume and not more than 10% by volume, preferably to be in the rangeof 1 to 8% by volume. Still more desirable results are obtained when thegas phase is displaced with such an inert gas as nitrogen, carbondioxide, or argon.

The temperature at which the molten maleimide is handled is to exceedthe melting point of the maleimide. From the viewpoint of minimizing thecoloration, this temperature is selected in the range lower by 1° to 50°C., preferably 5° to 20° C., than the melting point of the maleimide.

Now, the present invention will be described more specifically belowwith reference to working examples.

EXAMPLES 1 TO 35 AND CONTROLS 1 TO 3

In a hermetically sealable stainless steel vessel, a various maleimideand a various stabilizer metered out in various amounts were placed and,in a gas phase having a various molecular oxygen content, heated asimmersed in an oil bath kept at a prescribed temperature. In this case,the gas phase was displaced with nitrogen gas so as to adjust themolecular oxygen content thereof to a stated level. After the elapse ofa varying period in the range of one to three months, the vessel wasremoved from the oil bath and the solution consequently formed thereinwas visually examined as to the appearance. The maleimide resulting fromthe heating was subjected to solution polymerization with styrene andthe produced polymer was visually examined as to the appearance. Theresults are shown in Table 1 (the data of effects of alkyl-substitutedhydroxybenzenes used as stabilizer), Table 2 (the data of effects ofthiodipropionic esters used as stabilizer), Table 3 (the data of effectsof hindered phenols used as stabilizer), and Table 4 (the data forcomparison).

                                      TABLE 1                                     __________________________________________________________________________                       Amount of                                                                           Conc. of O.sub.2                                                                     Tem- Duration of                                                                         Appearance                                                                            Appearance                                                                           Appear-             Example            stabilizer                                                                          in gas phase                                                                         perature                                                                           heating                                                                             of maleimide                                                                          of maleimide                                                                         ance of             No.  Maleimides                                                                           Stabilizer                                                                           (ppm) (vol %)                                                                              (°C.)                                                                       (month)                                                                             before heating                                                                        after                                                                                polymer             __________________________________________________________________________    1    N-phenyl                                                                             2,4-dimetyl-6-                                                                       100   0.1    100  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            t-butyl phenol                 transparent                                                                           transparent                2    N-phenyl                                                                             2,4-dimetyl-6-                                                                       100   2      110  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            t-butyl phenol                 transparent                                                                           transparent                3    N-phenyl                                                                             4-t-butyl                                                                            100   2      110  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            cathecol                       transparent                                                                           transparent                4    N-phenyl                                                                             4,4'-thiobis                                                                         100   2      110  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            (6-t-butyl-                    transparent                                                                           transparent                            m-cresol)                                                         5    N-methyl                                                                             4-t-butyl                                                                            100   2      105  3     colorless                                                                             colorless                                                                            white                    maleimide                                                                            cathecol                       transparent                                                                           transparent                6    N-dodecyl                                                                            4-t-butyl                                                                            100   2       70  3     colorless                                                                             colorless                                                                            white                    maleimide                                                                            cathecol                       transparent                                                                           transparent                7    N-cyclohexyl                                                                         4-t-butyl                                                                            100   2      100  3     colorless                                                                             colorless                                                                            white                    maleimide                                                                            cathecol                       transparent                                                                           transparent                8    N-o-chloro                                                                           4-t-butyl                                                                            100   2       80  3     slightly                                                                              slightly                                                                             white                    phenyl cathecol                       yellowish                                                                             yellowish                       maleimide                             transparent                                                                           transparent                9    N-o-methyl                                                                           4-t-butyl                                                                            100   2       80  3     slightly                                                                              slightly                                                                             white                    phenyl cathecol                       yellowish                                                                             yellowish                       maleimide                             transparent                                                                           transparent                10   N-phenyl                                                                             4-t-butyl                                                                             10   2       80  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            cathecol                       transparent                                                                           transparent                11   N-phenyl                                                                             4-t-butyl                                                                            1000  2      100  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            cathecol                       transparent                                                                           transparent                12   N-phenyl                                                                             4-t-butyl                                                                            100   10     100  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            cathecol                       transparent                                                                           transparent                __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                       Amount of                                                                           Conc. of O.sub.2                                                                     Tem- Duration of                                                                         Appearance                                                                            Appearance                                                                           Appear-             Example            stabilizer                                                                          in gas phase                                                                         perature                                                                           heating                                                                             of maleimide                                                                          of maleimide                                                                         ance of             No.  Maleimides                                                                           Stabilizer                                                                           (ppm) (vol %)                                                                              (°C.)                                                                       (month)                                                                             before heating                                                                        after                                                                                polymer             __________________________________________________________________________    13   N-phenyl                                                                             ditridecyl-                                                                          100   0.1    100  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            3,3'-thiodi-                   transparent                                                                           transparent                            propionate                                                        14   N-phenyl                                                                             ditridecyl-                                                                          100   2      110  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            3,3'-thiodi-                   transparent                                                                           transparent                            propionate                                                        15   N-phenyl                                                                             dilauryl-3,3'                                                                        100   2      110  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            thiodi-                        transparent                                                                           transparent                            propionate                                                        16   N-methyl                                                                             distearyl-3,3'-                                                                      100   2      105  3     colorless                                                                             colorless                                                                            white                    maleimide                                                                            thiodi-                        transparent                                                                           transparent                            propionate                                                        17   N-dodecyl                                                                            distearyl-3,3'-                                                                      100   2       70  3     colorless                                                                             colorless                                                                            white                    maleimide                                                                            thiodi-                        transparent                                                                           transparent                            propionate                                                        18   N-cyclohexyl                                                                         distearyl-3,3'-                                                                      100   2      100  3     colorless                                                                             colorless                                                                            white                    maleimide                                                                            thiodi-                        transparent                                                                           transparent                            propionate                                                        19   N-o-chloro                                                                           distearyl-3,3'-                                                                      100   2       80  3     slightly                                                                              slightly                                                                             white                    phenyl thiodi-                        yellowish                                                                             yellowish                       maleimide                                                                            propionate                     transparent                                                                           transparent                20   N-o-methyl                                                                           distearyl-3,3'-                                                                      100   2       80  3     slightly                                                                              slightly                                                                             white                    phenyl thiodi-                        yellowish                                                                             yellowish                       maleimide                                                                            propionate                     transparent                                                                           transparent                21   N-phenyl                                                                             distearyl-3,3'-                                                                       10   2       80  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            thiodi-                        transparent                                                                           transparent                            propionate                                                        22   N-phenyl                                                                             distearyl-3,3'-                                                                      1000  2      100  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            thiodi-                        transparent                                                                           transparent                            propionate                                                        23   N-phenyl                                                                             distearyl-3,3'-                                                                      100   10     100  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            thiodi-                        transparent                                                                           transparent                            propionate                                                        __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                       Amount of                                                                           Conc. of O.sub.2                                                                     Tem- Duration of                                                                         Appearance                                                                            Appearance                                                                           Appear-             Example            stabilizer                                                                          in gas phase                                                                         perature                                                                           heating                                                                             of maleimide                                                                          of maleimide                                                                         ance of             No.  Maleimides                                                                           Stabilizer                                                                           (ppm) (vol %)                                                                              (°C.)                                                                       (month)                                                                             before heating                                                                        after                                                                                polymer             __________________________________________________________________________    24   N-phenyl                                                                             2,2'-thiobis(4-                                                                      100   0.1    100  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            methyl-6-t-                    transparent                                                                           transparent                            butyl phenol)                                                     25   N-phenyl                                                                             2,2'-thiobis(4-                                                                      100   2      110  3     yellowish                                                                             yellowish                                                                            white                    maleimide                                                                            methyl-6-t-                    transparent                                                                           transparent                            butyl phenol)                                                     26   N-phenyl                                                                             BOHT   100   2      110  3     yellowish                                                                             yellowish                                                                            white                    maleimide                             transparent                                                                           transparent                27   N-phenyl                                                                             TGBP   100   2      110  3     yellowish                                                                             yellowish                                                                            white                    maleimide                             transparent                                                                           transparent                28   N-methyl                                                                             "      100   2      105  3     colorless                                                                             colorless                                                                            white                    maleimide                             transparent                                                                           transparent                29   N-dodecyl                                                                            "      100   2       70  3     colorless                                                                             colorless                                                                            white                    maleimide                             transparent                                                                           transparent                30   N-cyclohexyl                                                                         "      100   2      100  3     colorless                                                                             colorless                                                                            white                    maleimide                             transparent                                                                           transparent                31   N-o-chloro                                                                           "      100   2       80  3     slightly                                                                              slightly                                                                             white                    phenyl                                yellowish                                                                             yellowish                       maleimide                             transparent                                                                           transparent                32   N-o-methyl                                                                           "      100   2       80  3     slightly                                                                              slightly                                                                             white                    phenyl                                yellowish                                                                             yellowish                       maleimide                             transparent                                                                           transparent                33   N-phenyl                                                                             "       10   2       80  3     yellowish                                                                             yellowish                                                                            white                    maleimide                             transparent                                                                           transparent                34   N-phenyl                                                                             "      1000  2      100  3     yellowish                                                                             yellowish                                                                            white                    maleimide                             transparent                                                                           transparent                35   N-phenyl                                                                             "      100   10     100  3     yellowish                                                                             yellowish                                                                            white                    maleimide                             transparent                                                                           transparent                __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________                       Amount of                                                                           Conc. of O.sub.2                                                                     Tem- Duration of                                                                         Appearance                                                                            Appearance                                                                           Appear-             Control            stabilizer                                                                          in gas phase                                                                         perature                                                                           heating                                                                             of maleimide                                                                          of maleimide                                                                         ance of             No.  Maleimides                                                                           Stabilizer                                                                           (ppm) (vol %)                                                                              (°C.)                                                                       (month)                                                                             before heating                                                                        after                                                                                polymer             __________________________________________________________________________    1    N-phenyl                                                                             none    0     2     100  1     yellowish                                                                             orange                                                                               --sin                    maleimide                             transparent                        2    N-phenyl                                                                             4-t-butyl                                                                            100   15     100  3     yellowish                                                                             orange slight                   maleimide                                                                            cathecol                       transparent                                                                           transparent                                                                          yellow              3    N-phenyl                                                                             4-t-butyl                                                                            100   21     100  3     yellowish                                                                             slightly                                                                             slight                   maleimide                                                                            cathecol                       transparent                                                                           brownish                                                                             yellowish                                                              transparent                                                                          brown               __________________________________________________________________________

EXAMPLES 36 TO 47

In a test tube, a various maleimide and a various stabilizer in varyingamounts were sealed in and heated as immersed in an oil bath at 150° C.for three hours (all of maleimides were molten state). In this case,nitrogen gas was used to form the gas phase (with the molecular oxygencontent adjusted to 5% by volume) in the test tube). After the threehours' heating, the sealed test tube was removed from the oil bath. Themaleimide in the sealed test tube was cooled and pulverized and thenanalyzed by high-speed liquid chromatography to determine purity. Theresults are shown in Table 5.

EXAMPLE 48

In a stainless steel vessel measuring 10 cm in diameter and 20 cm inheight and provided with a heating jacket, 1 kg of yellow N-phenylmaleimide having a purity of 99.8 f % by weight and 0.1 g of distearylpentaerythritol diphosphite were heated to a temperature of 100° C. withthe heat applied thereto through the jacket. In this case, the N-phenylmaleimide was a yellow liquid. The gas phase in the vessel was displacedwith nitrogen gas (with the molecular oxygen content adjusted to 8% byvolume).

The contents of the vessel were kept in the existent state for 30 days.At the end of the 30 days' standing, the N-phenyl maleimide was found tobe still retaining its original yellow color. On analysis by high-speedliquid chromatography, it was found to possess a purity of 99.6% byweight, a value representing practically no change from the originalpurity.

                                      TABLE 5                                     __________________________________________________________________________                                Amount of                                                                           Purity before                                                                        Purity after                         Example                     stabilizer                                                                          heating                                                                              heating                                                                             Appearance                                                                            Appearance             No.  Maleimide  Stabilizer  (ppm) (wt. %)                                                                              (wt. %)                                                                             before heating                                                                        after                  __________________________________________________________________________                                                           heating                36   N-phenyl maleimide                                                                       tris(2-chloroethyl)                                                                       100   99.8   99.5  yellow crystal                                                                        yellow crystal                         phosphate                                                     37   N-phenyl maleimide                                                                       distearyl pentaerithritol                                                                 100   99.8   99.6  "       "                                      diphosphite                                                   38   N-phenyl maleimide                                                                       tris nonylphenyl                                                                          100   99.8   99.6  "       "                                      phosphite                                                     39   N-phenyl maleimide                                                                       tristearyl phosphite                                                                      100   99.8   99.6  "       "                      40   N-methyl maleimide                                                                       distearyl pentaerithritol                                                                 100   99.8   99.6  white crystal                                                                         white crystal                          diphosphite                                                   41   N-dodecyl maleimide                                                                      distearyl pentaerithritol                                                                 100   98.7   98.3  "       "                                      diphosphite                                                   42   N-cyclohexyl                                                                             distearyl pentaerithritol                                                                 100   99.5   99.2  "       "                           maleimide  diphosphite                                                   43   N-o-chloro phenyl                                                                        distearyl pentaerithritol                                                                 100   98.9   98.6  slightly                                                                              slightly yellow             maleimide  diphosphite                    crystal crystal                44   N-o-methyl phenyl                                                                        distearyl pentaerithritol                                                                 100   99.5   99.3  slightly                                                                              slightly yellow             maleimide  diphosphite                    crystal crystal                45   N-phenyl maleimide                                                                       distearyl pentaerithritol                                                                  10   99.8   99.0  yellow crystal                                                                        yellow crystal                         diphosphite                                                   46   N-phenyl maleimide                                                                       distearyl pentaerithritol                                                                 1000  99.8   99.6  "       "                                      diphosphite                                                   47   N-phenyl maleimide                                                                       distearyl pentaerithritol                                                                 10000 99.8   99.6  "       "                                      diphosphite                                                   __________________________________________________________________________

EXAMPLES 48 TO 61

In a hermetically sealable stainless steel vessel, a various maleimideand a various stabilizer in varying amounts were placed and, in a gasphase having a various molecular oxygen content, heated as immersed inan oil bath kept at a prescribed temperature. In this case, the gasphase in the vessel was displaced with nitrogen gas with the molecularoxygen content therein adjusted to a stated level. After the elapse ofthree months, the vessel was removed from the oil bath and the solutionconsequently formed therein was visually examined as to the appearance.The maleimide resulting from the heating was subjected to solutionpolymerization with styrene. The produced polymer was visually examinedas to the appearance. The results are shown in Table 6.

                                      TABLE 6                                     __________________________________________________________________________    Ex-                    Amount of                                                                             Conc. of                                                                           Tem-                                                                             Duration                                                                           Appearance                        am-                    stabilizer                                                                            O.sub.2 in gas                                                                     per-                                                                             of   of maleimide                                                                         Appearance                                                                           Appear-             ple      Stabilizer    (1) (2) phase                                                                              ature                                                                            heating                                                                            before of maleimide                                                                         ance of             No.                                                                              Maleimide                                                                           (1)    (2)    (ppm)                                                                             (ppm)                                                                             (vol %)                                                                            (°C.)                                                                     (month)                                                                            heating                                                                              after                                                                                polymer             __________________________________________________________________________    48 N-phenyl                                                                            4-t-butyl                                                                            tris(2-                                                                              100 500 0.1  100                                                                              3    yellowish                                                                            yellowish                                                                            white                  maleimide                                                                           cathecol                                                                             chloroethyl)                transparent                                                                          transparent                                phosphate                                                     49 N-phenyl                                                                            4,4'-thiobis                                                                         trisnonyl                                                                            100 500 5    110                                                                              3    yellowish                                                                            yellowish                                                                            white                  maleimide                                                                           (6-t-butyl-                                                                          phenol                      transparent                                                                          transparent                         m-cresol)                                                                            phosphite                                                     50 N-phenyl                                                                            2,5-di-t-butyl                                                                       pentaeri-                                                                            100 500 5    110                                                                              3    yellowish                                                                            yellowish                                                                            white                  maleimide                                                                           hydroquinone                                                                         thritol                     transparent                                                                          transparent                                diphosphite                                                   51 N-phenyl                                                                            BOHT (*)                                                                             pentaeri-                                                                            1000                                                                              3000                                                                              5    110                                                                              3    yellowish                                                                            yellowish                                                                            white                  maleimide    thritol                     transparent                                                                          transparent                                diphosphite                                                   52 N-phenyl                                                                            TGBP (**)                                                                            pentaeri-                                                                            1000                                                                              3000                                                                              5    110                                                                              3    yellowish                                                                            yellowish                                                                            white                  maleimide    thritol                     transparent                                                                          transparent                                diphosphite                                                   53 N-methyl                                                                            4-t-butyl                                                                            pentaeri-                                                                            100 500 5    105                                                                              3    yellowish                                                                            yellowish                                                                            white                  maleimide                                                                           cathecol                                                                             thritol                     transparent                                                                          transparent                                diphosphite                                                   54 N-dodecyl                                                                           4-t-butyl                                                                            pentaeri-                                                                            100 500 5     70                                                                              3    colorless                                                                            colorless                                                                            white                  maleimide                                                                           cathecol                                                                             thritol                     transparent                                                                          transparent                                diphosphite                                                   55 N-cyclo-                                                                            4-t-butyl                                                                            pentaeri-                                                                            100 500 5    100                                                                              3    colorless                                                                            colorless                                                                            white                  hexyl cathecol                                                                             thritol                     transparent                                                                          transparent                   maleimide    diphosphite                                                   56 N-o-chloro                                                                          4-t-butyl                                                                            pentaeri-                                                                            100 500 5     80                                                                              3    colorless                                                                            colorless                                                                            white                  phenyl                                                                              cathecol                                                                             thritol                     transparent                                                                          transparent                   maleimide    diphosphite                                                   57 N-o-methyl                                                                          4-t-butyl                                                                            pentaeri-                                                                            100 500 5     80                                                                              3    slightly                                                                             slightly                                                                             white                  phenyl                                                                              cathecol                                                                             thritol                     yellowish                                                                            yellowish                     maleimide    diphosphite                 transparent                                                                          transparent                58 N-phenyl                                                                            4-t-butyl                                                                            pentaeri-                                                                            100 500 5     80                                                                              3    slightly                                                                             slightly                                                                             white                  maleimide                                                                           cathecol                                                                             thritol                     yellowish                                                                            yellowish                                  diphosphite                 transparent                                                                          transparent                59 N-phenyl                                                                            4-t-butyl                                                                            pentaeri-                                                                             10 100 5    100                                                                              3    yellowish                                                                            yellowish                                                                            white                  maleimide                                                                           cathecol                                                                             thritol                     transparent                                                                          transparent                                diphosphite                                                   60 N-phenyl                                                                            4-t-butyl                                                                            pentaeri-                                                                            1000                                                                              3000                                                                              5    110                                                                              3    yellowish                                                                            yellowish                                                                            white                  maleimide                                                                           cathecol                                                                             thritol                     transparent                                                                          transparent                                diphosphite                                                   61 N-phenyl                                                                            4-t-butyl                                                                            pentaeri-                                                                            100 500 10   110                                                                              3    yellowish                                                                            yellowish                                                                            white                  maleimide                                                                           cathecol                                                                             thritol                     transparent                                                                          transparent                                diphosphite                                                   __________________________________________________________________________     (*) BOHT:                                                                     2,4bis(n-octylthio)-6-(hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine          (**) TGBP: triethylene                                                        glycolbis-[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate]-            

EXAMPLE 62

A hermetically sealable cylindrical tank (5.6 m in length, 2.2 m ininside diameter, and 20 m³ in inner volume) provided with a heating partwas charged with 18 tons of molten N-phenyl maleimide having a purity of99.6% by weight and containing 0.01% by weight of triethylene glycolbis-[3-(3-t-butyl-5-methyl-4-hydroxyphenyl) propionate] and 0.05% byweight of tristearyl phosphite each as a stabilier. Thereafter, the gasphase in the tank was displaced with nitrogen gas so as to acquire amolecular oxygen content of 5% by volume and the liquid phase wassolidified by spontaneous cooling.

After the contents of the tank were retained in the existent state forone month, they were melted again by passing steam at 110° C. throughthe heating part. The resultant solution was found to be a yellowtransparent liquid showing no sign of the occurrence of polymerization.On analysis by liquid chromatography, this solution was found to possessa purity of 99.6%, a value indicating perfect absence of change. Whenthe same N-phenyl maleimide and styrene were subjected to solutionpolymerization by the procedure described in Referential Example, therewas obtained a white polymer showing no sign of coloration.

EXAMPLE 63

The procedure of Example 62 was faithfully repeated, except that 0.005%by weight of 4-tert-butyl catechol and 0.05% by weight of distearylpentaerythritol diphosphite were used instead as stabilizers and themolecular oxygen content in the gas phase was adjusted by nitrogen gasdisplacement to 10% by volume. When the produced N-phenyl maleimide wasmelted again, there was obtained a yellow clear liquid showing no signof polymerization. The results were perfectly equal to those obtained inExample 62.

REFERENTIAL EXAMPLE (SYNTHESIS OF POLYMER)

A four-necked flask having an inner volume of 1 liter and provided witha stirrer, a reflux condenser, a nitrogen gas inlet, and a droppingfunnel was charged with 44 g of methylethyl ketone. The flask, with thegas phase therein thoroughly displaced with nitrogen, was heated to 80°C. To the methylethyl ketone kept heated at an inner temperature of 80°C., a mixed solution comprising of 96.55 g of N-phenyl maleimide, 58.07g of styrene, and 460 g of methylethyl ketone and a solution consistingof 0.77 g of azoisobutylonitrile and 11 g of methylethyl ketone bothprepared separately were added dropwise over a period of four hours. Theresultant mixture was continuously stirred for one hour. Then, thereaction product consequently formed was cooled and transferred into twoliters of methanol, separated by filtration, and dried, to obtain 147.8g of a polymer. This polymer was a white powdery substance and was foundto possess a molecular weight of 100,000 (by GPC analysis). Polymerswere synthesized by following the same procedure, excepting the molarratio of a varying maleimide to styrene was fixed at 1 and the amount ofazoisobutylonitrile was fixed at 0.5% by weight based on the amount ofmonomer. These polymers were evaluated as to appearance.

What is claimed is:
 1. A method for handling a maleimide, which methodcomprises handling said maleimide in the presence of a stabilizer at atemperature exceeding the melting point thereof in a state kept incontact with a gas phase having a molecular oxygen content of not morethan 10% by volume.
 2. A method according to claim 1, wherein saidstabilizer is at least one compound selected from the group consistingof alkyl-substituted hydroxybenzenes, thiodipropionic esters, hinderedphenols, phosphoric esters, phosphorous esters, and phosphoric acidamides.
 3. A method according to claim 2, wherein the amount of saidstabilizer is in the range of 0.0001 to 1% by weight based on the amountof said maleimide.
 4. A method according to claim 1, wherein themolecular oxygen content in said gas phase is in the range of 0.1 to 10%by volume.
 5. A method according to claim 1, wherein the temperature atwhich said maleimide is handled is not lower than the melting point ofsaid maleimide and not higher than the sum of said melting point and 50°C.
 6. A method according to claim 2, wherein said stabilizer is analkyl-substituted hydroxybenzene.
 7. A method according to claim 2,wherein said stabilizer is a thiodipropionic ester.
 8. A methodaccording to claim 2, wherein said stabilizer is a hindered phenol.
 9. Amethod according to claim 2, wherein said stabilizer is at least onephosphorus compound selected from the group consisting of phosphoricesters, phosphorous esters, and phosphoric acid amides.
 10. A methodaccording to claim 9, wherein said phosphorus compound is at least onemember selected from the group consisting of tris(2-chloroethyl)phosphate, tris-nonylphenyl phosphite, tristearyl phosphite, distearylpentaerythritol diphosphite, and hexamethyl phosphoric triamide.
 11. Amethod according to claim 2, wherein said stabilizer is a mixtureconsisting of at least one alkyl-substituted hydroxybenzene and at leastone phosphorus compound selected from the group consisting of phosphoricesters, phosphorous esters, and phosphoric acid amides.
 12. A methodaccording to claim 11, wherein said alkyl-substituted hydroxybenzene isat least one member selected from the group consisting of2,4-dimethyl-6-tert-butyl phenol, 4-tert-butyl catechol,2,4-di-tert-butyl hydroquinone, 2-tert-butyl hydroquinone,4,4'-thio-bis(6-tert-butyl-m-cresol),2,4-bis(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-tiazine,2,2'-thio-bis-(4-methyl-6-t-butylphenol), and triethyleneglycol-bis-[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate].
 13. Amethod according to claim 11, wherein said phosphorous ester and saidphosphoric ester are at least one member selected from the groupconsisting of tris(nonylphenyl) phosphite, tristearyl phosphite,distearyl pentaerythritol diphosphite, and tris(2-chloroethyl)phosphate.
 14. A method for handling a maleimide, which method comprisesmelting said maleimide in the presence of a stabilizer at a temperatureexceeding the melting point thereof in a state kept in contact with agas phase having a molecular oxygen content of not more than 10% byvolume, cooling the resultant molten maleimide until solidification,heating the solid maleimide in the presence of said stabilizer at atemperature exceeding said melting point in a state kept in contact witha gas phase having a molecular oxygen content of not smaller than 0.1%by volume, and handling the resultant liquid maleimide.
 15. A methodaccording to claim 14, wherein said stabilizer is at least one compoundselected from the group consisting of alkyl-substituted hydroxybenzenes,thiodipropionic esters, hindered phenols, phosphoric esters, phosphorousesters, and phosphoric acid amides.
 16. A method according to claim 14,wherein the amount of said stabilizer is in the range of 0.0001 to 1% byweight based on the amount of said maleimide.
 17. A method according toclaim 14, wherein the molecular oxygen content in said gas phase ineither of said heat treatments is in the range of 0.1 to 10% by volume.18. A method according to claim 14, wherein the temperature at whichsaid maleimide is handled is not lower than the melting point of saidmaleimide and not higher than the sum of said melting point and 50° C.19. A method according to claim 14 wherein said solid maleimide has beenobtained by placing the maleimide in a liquid state in a hermeticallysealable container said container being provided with a part to beheated, and then cooling said liquid maleimide until solidification.